The present invention relates to an improved electrolyte composition and process for electrodepositing a composite nickel containing electroplate on a corrosion susceptible base metal to achieve corrosion protection thereof. The composite electroplate comprises three adjacent, bonded nickel-containing layers each of a controlled thickness and controlled sulfur content which normally are provided with a conventional chrome plate over the surface of the outer nickel layer achieving exceptional outdoor corrosion protection of the basis metal in comparison to a single or even a duplex nickel-containing electroplate of the same thickness. Such composite nickel-containing electroplates are in widespread commercial use for protecting basis metals such as steel, copper, brass, aluminum or zinc die castings which are subject to outdoor exposure during service, particularly, to marine and automotive service conditions. Beneficial results in corrosion protection are also achieved in the use of such composite nickel-containing layers on plastic substrates which have been subjected to a suitable pretreatment in accordance with well-known techniques to provide an electrically conductive coating thereover such as a copper layer rendering the plastic substrate receptive to nickel electroplating. Typical of such plastic materials which can be electroplated are ABS, polyolefin, polyvinylchloride, and phenolformaldehyde polymers. Such composite nickel-containing electroplates when used in connection with plastic substrates substantially reduce or eliminates so-called "green" corrosion stains produced by a corrosive attack of a copper basis layer or flash.
Typical of such prior art composite nickel-containing electroplating processes and compositions are those disclosed in U.S. Pat. Nos. 3,090,733 and 3,703,448 the substance of which are incorporated herein by reference. In accordance with U.S. Pat. No. 3,090,733 issued May 21, 1963 a process is disclosed for electrodepositing a three-layered nickel-containing deposit on a substrate in which at least the operating bath for applying the intermediate nickel layer contains selected sulfur compounds to effect a controlled sulfur content in the intermediate nickel-containing layer to achieve the requisite adherence between the composite layers and corrosion protection of the underlying substrate. A further improvement in the foregoing process is disclosed in U.S. Pat. No. 3,703,448 issued Nov. 21, 1972 in which alternative sulfur compounds comprising thiosulfonates of nitriles or amides are employed at least in the operating bath for electrodepositing the intermediate layer.
The composition and process of the present invention provides for still further improvements over the compositions and processes disclosed in the aforementioned two patents employing a novel sulfur compound at least in the operating bath for electrodepositing the intermediate layer which provides for improved bath stability in the presence of air agitation, high temperature and low pH providing for increased plating speeds and reduced consumption of the additive compound. The novel sulfur additive compound of this invention provides the further advantages in that it can readily be analyzed in the operating bath to maintain its concentration within the optimum operating range and contamination of the operating bath for applying the outer nickel-containing layer with the sulfur additive compound by drag-in from the intermediate layer operating bath does not appreciably effect the sulfur concentration of the outer nickel-containing layer. This latter advantage is important because normally a water rinse step is not employed between the intermediate and outer nickel plating steps and an undesirable increase in sulfur content of the outer nickel layer can in some instances result in hindrance of coverage of the final chromium electrodeposit.